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18-04.md
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------------------------ --------------------------------- --------------------
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[Previous](18-03.html) [Table of Contents](index.html) [Next](18-05.html)
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------------------------ --------------------------------- --------------------
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**LISTING 18.3 MAIN.C**
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// MAIN.C
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//
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// David Stafford
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//
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#include <stdlib.h>
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#include <stdio.h>
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#include <conio.h>
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#include <time.h>
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#include <bios.h>
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#include “life.h”
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// functions in VIDEO.C
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void enter_display_mode( void );
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void exit_display_mode( void );
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void show_text( int x, int y, char *text );
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void InitCellmap( void )
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{
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unsigned int i, j, t, x, y, init;
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for( init = (HEIGHT * WIDTH * 3) / 2; init; init— )
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{
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x = random( WIDTH * 3 );
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y = random( HEIGHT );
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CellMap[ (y * WIDTH) + x / 3 ] |= 0x1000 << (2 - (x % 3));
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}
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for( i = j = 0; i < WIDTH * HEIGHT; i++ )
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{
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if( CellMap[ i ] & 0x7000 )
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{
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ChangeList1[ j++ ] = (short)&CellMap[ i ];
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}
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}
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NextGen(); // Set cell states, prime the pump.
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}
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void main( void )
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{
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unsigned long generation = 0;
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char gen_text[ 80 ];
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long start_time, end_time;
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unsigned int seed;
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printf( “Seed (0 for random seed): ” );
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scanf( “%d”, &seed );
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if( seed == 0 ) seed = (unsigned) time(NULL);
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srand( seed );
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#ifndef NODRAW
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enter_display_mode();
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show_text( 0, 10, “Generation:” );
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#endif
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InitCellmap(); // randomly initialize cell map
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_bios_timeofday( _TIME_GETCLOCK, &start_time );
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do
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{
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NextGen();
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generation++;
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#ifndef NOCOUNTER
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sprintf( gen_text, “%10lu”, generation );
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show_text( 0, 12, gen_text );
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#endif
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}
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#ifdef GEN
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while( generation < GEN );
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#else
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while( !kbhit() );
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#endif
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_bios_timeofday( _TIME_GETCLOCK, &end_time );
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end_time -= start_time;
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#ifndef NODRAW
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getch(); // clear keypress
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exit_display_mode();
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#endif
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printf( “Total generations: %ld\nSeed: %u\n”, generation, seed );
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printf( “%ld ticks\n”, end_time );
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printf( “Time: %f generations/second\n”,
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(double)generation / (double)end_time * 18.2 );
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}
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**LISTING 18.4 VIDEO.C**
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/* VGA mode 13h functions for Game of Life.
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Tested with Borland C++. */
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#include <stdio.h>
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#include <conio.h>
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#include <dos.h>
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#define TEXT_X_OFFSET 28
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#define SCREEN_WIDTH_IN_BYTES 320
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#define SCREEN_SEGMENT 0xA000
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/* Mode 13h mode-set function. */
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void enter_display_mode()
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{
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union REGS regset;
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regset.x.ax = 0x0013;
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int86(0x10, ®set, ®set);
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}
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/* Text mode mode-set function. */
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void exit_display_mode()
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{
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union REGS regset;
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regset.x.ax = 0x0003;
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int86(0x10, ®set, ®set);
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}
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/* Text display function. Offsets text to non-graphics area of
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screen. */
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void show_text(int x, int y, char *text)
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{
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gotoxy(TEXT_X_OFFSET + x, y);
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puts(text);
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}
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**LISTING 18.5 LIFE.H**
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void far NextGen( void );
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extern unsigned short CellMap[];
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extern unsigned short far ChangeList1[];
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#define LEFT (-2)
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#define RIGHT (+2)
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#define UP (WIDTH * LEFT)
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#define DOWN (WIDTH * RIGHT)
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#define UPPERLEFT (UP + LEFT)
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#define UPPERRIGHT (UP + RIGHT)
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#define LOWERLEFT (DOWN + LEFT)
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#define LOWERRIGHT (DOWN + RIGHT)
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#define WRAPLEFT (RIGHT * (WIDTH - 1))
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#define WRAPRIGHT (LEFT * (WIDTH - 1))
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#define WRAPUP (DOWN * (HEIGHT - 1))
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#define WRAPDOWN (UP * (HEIGHT - 1))
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### Keeping Track of Change with a Change List {#Heading5}
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In my earlier optimizations to the Game of Life, described in the last
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chapter, I noted that most cells in a Life cellmap are dead, and in most
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cases all the neighbors are dead as well. This observation enabled me to
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get a major speed-up by scanning the cellmap for the few non-zero bytes
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(cells that were either alive or have neighbors that are alive).
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Although that was a big improvement, it still required my code to touch
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every cell to check its state. David has improved on this by maintaining
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a *change list;* that is, a list of pointers to cells that change in the
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current generation. Only those cells and their neighbors need to be
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checked or touched in any way in order to create the next generation,
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saving a great many instructions and also a great many cache misses due
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to the fact that cellmaps are too big to fit into the 486’s internal
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cache. During a given generation, David runs down the list of cells that
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changed from the previous generation to make the changes for this
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generation, and in the process generates the change list for the next
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generation.
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That’s the overall approach, but this being David Stafford, it’s not
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that simple, of course. I’ll let him tell you how his implementation
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works in his own words. (I’ve edited David’s text a bit, and added my
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own comments in square brackets, so blame me for any errors.)
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“Each three cells in the life grid are packed into two bytes, as shown
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in Figure 18.1. So, it is convenient if the width of the cell array is
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an even multiple of three. There’s nothing in the algorithm that
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prevents it from supporting any arbitrary size, but the code is a bit
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simpler this way. So if you want a 200x200 grid, I recommend just using
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a 201x200 grid, and be happy with the extra free column. Otherwise the
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edge wrapping code gets more complex.
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“Since every cell has from zero to eight neighbors, you may be wondering
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how I can manage to keep track of them with only three bits. Each cell
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really has only a maximum of seven neighbors since we only need to keep
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track of neighbors *outside* of the current cell word. That is, if cell
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‘B’ changes state then we don’t need to reflect this in the neighbor
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counts of cells ‘A’ and ‘C.’ Updating is made a little faster. [In other
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words, when David picks up a word representing three cells, each of the
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three cells has at least one of the other cells in that word as a
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neighbor, and the state of that neighbor is stored right in that word,
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as shown in Figure 18.1. Therefore, the neighbor count for a given cell
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never needs to reflect more than seven neighbors, because at least one
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of the eight neighbors’ states is already encoded in the word.]
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\
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**Figure 18.1** *Cell triplet storage.*
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------------------------ --------------------------------- --------------------
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[Previous](18-03.html) [Table of Contents](index.html) [Next](18-05.html)
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------------------------ --------------------------------- --------------------
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* * * * *
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Graphics Programming Black Book © 2001 Michael Abrash
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